Fix crash during debug when escaping pterabyte

When compiled with DEBUGMODE=1, the game crashes when a player escapes a Pterabyte. This is because of a sanity check that makes sure that P_SetMobjState is not called on a player, which is was in this case. I don't know what effects this has on a release build, but since P_SetMobjState shouldn't be called on a player, I'd say it's better to just fix it.